Hydrogen fuel cells have been attracting attention as an energy source capable of restraining emission of carbon dioxide. A hydrogen fuel cell has a fuel cell body (cell body) comprising fuel and air electrodes joined to an electrolyte, and with the temperature of the cell body kept at a predetermined power generation start temperature or above by heat applied, for example, from an external heater, a fuel gas is supplied to the cell body, whereby electric power is generated.
The cell body of a solid oxide fuel cell (SOFC) comprises an electrolyte of solid oxide, and a fuel electrode (hydrogen electrode) and an air electrode (oxygen electrode) both joined to (formed on) the solid oxide electrolyte. In the solid oxide fuel cell, the cell body is fed with a fuel gas and air, whereupon electric power is generated.
The solid oxide fuel cell is known to have a variety of advantages, as set forth in “Fuel Cell Technologies”, First Edition, Third Impression, pp. 183-230, by the Special Committee on Technological Investigation of Next-generation Systems for Electricity Generation by Fuel Cells, the Institute of Electrical Engineers of Japan, published from Ohmsha, Ltd. on Jan. 5, 2005 (hereinafter referred to as Document 1).
Specifically, with the solid oxide fuel cell, high output is available, and besides, not only hydrogen gas but gases containing large quantities of carbon monoxide can be used as fuel. Also, since the solid oxide fuel cell operates at high temperatures, it is unnecessary to use an expensive platinum catalyst and the internal reforming can be utilized. Further, hydrogen or carbon monoxide required for the cell reaction can be produced from the fuel, such as methane, by making use of heat generated by the cell body, and thus, the device equipped with the solid oxide fuel cell can be reduced in size and improved in efficiency.
It has been known from Document 1 and other publications that a dual-chamber-type solid oxide fuel cell is a common type in which the fuel and air electrodes are isolated from each other by a separator and are respectively supplied with a fuel gas and air or the like.
A single-chamber-type solid oxide fuel cell is also known wherein the fuel and air electrodes are not isolated by a separator and electric power is generated with the cell body placed in an atmosphere containing the mixture of a fuel gas, such as hydrogen or methane, and air or the like.
Such single-chamber-type solid oxide fuel cells are disclosed, for example, in Unexamined Japanese Patent Publications No. 2002-280015, No. 2002-280017, and No. 2002-313357.
Fuel cells are associated, however, with the problem of energy loss accompanying release of thermal energy. Especially in the case of solid oxide fuel cells that operate (generate electricity) in high-temperature environments, the electric power generation efficiency is considerably low. Also, it is very often the case that the heat of reaction (thermal energy) generated at the fuel and air electrodes is not effectively utilized but emitted to outside.